As a long time collector (30+ years) I learned long ago that no recordings is worth the inflated prices dealers try to charge for "rare" items.
We have seen some of the top rarities of the cast album field get reissued on CD including Jerry Herman's PARADE (once a $200 item on Lp) and the Ann Southern TV cast of LADY IN THE DARK. Since CD's can be easily copied, the rarity factor really only applies to hardcore collectors who must have the original release with all the packaging.
Judging by what I read on BWW and elsewhere, many younger listeners don't care about the package: they just want the music, and CD-R copies will suffice.
The top rarities right on CD now are WHOOP-UP (Polydor) and KWAMINA (Broadway Angel) and both were deleted within about 2 years of being released. KWAMINA has been rumored for reissue by DRG but no one wants to touch WHOOP-UP - even to make it available as a download. Other hard to get titles include the original London cast of CHILDEREN OF EDEN (a pressing defect has made most existing copies unplayable. IF anyone has a copy that does play you would be well advised to make a back-up copy.) Also the Judi Dench revival cast of A LITTLE NIGHT MUSIC was recorded under such a restricted agreement that it cannot be reissued and the label that did release it has gone out of business, but I have been told that the tracks are available on a budget CD but it is not called A LITTLE NIGHT MUSIC. (The title is vague something like "Judi Dench sings Sondheim")
Aside from this small handful of titles, most everything else can be found with a bit of searching on E-bay or Amazon or downloaded from Soulseek.
Cast albums are NOT "soundtracks."
Live theatre does not use a "soundtrack." If it did, it wouldn't be live theatre!
I host a weekly one-hour radio program featuring cast album selections as well as songs by cabaret, jazz and theatre artists. The program, FRONT ROW CENTRE is heard Sundays 9 to 10 am and also Saturdays from 8 to 9 am (eastern times) on www.proudfm.com